1. Field of the Invention
The present invention relates to a method for producing aromatic hydrocarbons from light hydrocarbons. More particularly, the present invention is concerned with a method for producing aromatic hydrocarbons from light hydrocarbons, which comprises supplying a light hydrocarbon feedstock comprising at least one member selected from the group consisting of olefins and paraffins to a fixed-bed, adiabatic reactor containing a fixed catalyst bed comprised of a zeolite catalyst, to thereby contact the light hydrocarbon feedstock with the zeolite catalyst in the fixed-bed, adiabatic reactor and effect a catalytic cyclization reaction of the light hydrocarbon feedstock, wherein the zeolite catalyst has a specific activity, and the catalytic cyclization reaction is conducted under specific temperature conditions with respect to the catalyst bed.
By the method of the present invention, not only can aromatic hydrocarbons be produced in high yield, but also a lowering of the catalytic activity is small, so that the production of aromatic hydrocarbons can be stably conducted for a prolonged period of time. The method of the present invention can be advantageously used in the petrochemical industry and in petroleum refining, especially in the production of aromatic compounds and high-octane gasolines.
2. Prior Art
Various methods are conventionally known in which aromatic hydrocarbons are produced using as a catalyst a zeolite, such as ZSM-5. For example, Examined Japanese Patent Application Publication No. 56-42639 (corresponding to U.S. Pat. No. 3,756,942) discloses a method in which aromatic hydrocarbons are produced from a hydrocarbon feedstock which is comprised of paraffins, olefins and/or naphthenes, each having 5 or more carbon atoms, and which has an aromatic hydrocarbon content of 15% by weight or less, using a ZSM-5 type zeolite catalyst. Examined Japanese Patent Application Publication No. 4-5712 discloses a method in which aromatic hydrocarbons are produced from a hydrocarbon feedstock containing saturated hydrocarbons having 4 or less carbon atoms, unsaturated hydrocarbons having 2 to 4 carbon atoms and a virgin naphtha in a specific amount ratio, using a ZSM-5 type zeolite catalyst.
Further, U.S. Pat. No. 3,845,150 discloses a process in which a hydrocarbon comprising 20 to 65% by weight of saturated hydrocarbons and 20 to 50% by weight of unsaturated hydrocarbons is contacted with a ZSM-5 type zeolite catalyst, so that the process involving the cyclization reaction (exothermic) of the unsaturated hydrocarbons and the cyclization reaction (endothermic) of the saturated hydrocarbons can be performed under heat balanced conditions, thereby producing aromatic hydrocarbons by an isothermal reaction.
Japanese Patent Application prior-to-examination Publication (kohyo) No. 3-503656 (corresponding to U.S. Pat. No. 4,851,602) discloses a method in which a hydrocarbon feedstock containing lower alkanes and lower alkenes is contacted with a fluidized bed of an acid type zeolite catalyst having medium pores in a first conversion zone, thereby obtaining a reaction mixture (effluent stream) containing aromatic hydrocarbon-rich higher aliphatic hydrocarbons, and the obtained reaction mixture is contacted with a fluidized bed of an acid type zeolite catalyst having medium pores in a second conversion zone, thereby obtaining a product which is rich in alkylated aromatic hydrocarbons and which contains gasolines having 5 or more carbon atoms.
Further, Unexamined Japanese Patent Application Laid-Open Specification No. 63-69888 (corresponding-to U.S. Pat. No. 4,720,602) discloses a method in which a hydrocarbon feedstock containing at least 50% by weight of C.sub.2 -C.sub.12 aliphatic hydrocarbons is converted to aromatic compounds, using a crystalline zeolite catalyst having a specific activity.
Unexamined Japanese Patent Application Laid-Open Specification No. 63-14732 discloses a method in which aromatic hydrocarbons are produced from light hydrocarbons, using a ZSM-5 type zeolite catalyst containing zinc and having specific properties.
Unexamined Japanese Patent Application Laid-Open Specification No. 3-182592 (corresponding to U.S. Pat. No. 4,885,420) discloses a method in which a hydrocarbon feedstock containing olefins is subjected to a hydrogenation reaction using hydrogen and a hydrogenation catalyst and then, the resultant product is subjected to a dehydrocyclodimerization reaction in a reactor containing a dehydrocyclodimerization catalyst, to thereby obtain aromatic hydrocarbons.
However, in the above-mentioned conventional methods, when it is attempted to produce aromatic hydrocarbons by using a fixed-bed, adiabatic reactor (which is commercially most advantageous because it is not only simple in structure but also has high efficiency), problems arise such that the yield of desired aromatic hydrocarbon product becomes low, or coking vigorously occurs, so that it becomes difficult to stably perform the process for the production of desired aromatic hydrocarbons. Therefore, it has conventionally been considered to be impossible to stably produce desired aromatic hydrocarbons in high yield by using a fixed-bed, adiabatic reactor. For producing aromatic hydrocarbons in high yield, some attempts have heretofore been made. For example, in Unexamined Japanese Patent Application Laid-Open Specification No. 3-182592, a process has been proposed in which olefins contained in a feedstock are first subjected to hydrogenation and then, the feedstock is subjected to a dehydrocyclodimerization reaction for the production of aromatic hydrocarbons. This process, however, is disadvantageous in that it needs to be carried out in two stages. Further, in some of the conventionally proposed methods for producing aromatic hydrocarbons in high yield, it is disadvantageously necessary to use reactors having complicated structures (such as an isothermal reactor, a moving-bed reactor and a fluidized-bed reactor).
As mentioned above, U.S. Pat. No. 3,845,150 discloses a process in which a feedstock containing saturated hydrocarbons and unsaturated hydrocarbons in a specific weight ratio is used so as to enable the process to be carried out under heat balanced conditions. In this process, although almost no heat is supplied from an external source, aromatic hydrocarbons are produced in substantially the same yield as in the case of a method in which a large amount of heat is supplied to the reaction system. However, in this prior art reference, there is no description concerning a temperature distribution in the reaction system or how to conduct a stable operation in which a lowering of the catalytic activity which is caused due to occurrence of coking on the catalyst, is suppressed. The above-mentioned Japanese Patent Application prior-to-examination Publication (kohyo) No. 3-103656 discloses a method in which a hydrocarbon feedstock containing lower alkanes and lower alkenes in such a weight ratio as to maintain almost isothermal reaction conditions in the conversion zone, is used to obtain a product which is rich in alkylated aromatic hydrocarbons and contains gasolines having 5 or more carbon atoms. In this method, a fluidized-bed reactor (in which both the catalytic reaction and the regeneration of the catalyst can be continuously conducted) is used to thereby prevent a lowering of the catalytic activity due to occurrence of coking on the catalyst. However, the fluidized-bed reactor used in this method has a complicated structure, so that the cost becomes high.